I flahed 6.2.18 and same issue..BUT ! I think it's my fault due to apps I uninstall/replace. Now I flashed 6.2.25 , restore my apps and settings ( with MIUI Backup app ) but not uninstalling/replacing any app and no problem with signal.
I ALWAYS delete/replace those apps în any MIUI Rom until now and I had no problem with anything :
Analytics ( Not sure if always )
Browser
Cloud Print
com.miui.mipub
Xiaomi Account
Print Spooler
DocumentUI
E-mail
File Explorer
MiGalleryLockscreen ( Not sure if always )
Find Device
Mms
MiCloud
MiCloud Service Framework
MiDrop
Miravision
MusixFX
Yellow Pages
PicoTTS
MiuiVideo
FM
BugReport
LiveWallpapersPicker
MiRemote
MiLink Service
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Voice Unlock ( From 6.2.25 I deleted folders /system/etc/voicecommand , voiceextension
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Also it's ok if I delete /sistem/etc/jeejen folder ?
Is there any app I shouldn't delete ? I personally don't see any app that should affect phone signal.. :/ like MiuiGallery, Video, File Explorer, etc..
Maybe they changed something in latest builds that affect signal if delete some apps.. ? :/